Significance of greentones for the formation of accretionaly complex of Japanese Islands during Jurassic to Paleogene

本研究项目的研究结果总结如下：重新研究了出露于索拉齐—叶佐带的绿岩的岩石学特征和赋存方式。以往认为具有N-MORB特征的绿岩，其微量元素、稀土元素、同位素组成等地球化学特征与海洋高原玄武岩相当相似。特别是Nb含量略高于N-MORB，而Y和Zr等HFS元素比N-MORB贫。此外，在多个地区发现了绿岩与燧石的侵入关系，表明火成岩活动是在远洋沉积物沉积后开始的。因此，这些侵入岩可能对应于岩浆活动的初始阶段，如热点型或海洋高原型岩浆活动，并对日本西南部美武带的绿岩进行了地球化学和年龄资料的研究。在冲绳岛至北海道的岛东带和飞隆带，沿日本全岛广泛地描述了具有MORB特征的原位绿岩，这些绿岩可能是洋脊俯冲产生的。研究了它们的年龄测定、赋存方式、岩石学和岩石学特征。与原位绿岩相关的沉积物中放射虫纪化石时代向北的系统年轻化逐渐清晰。此外，还明确了玄武岩与侵入沉积物之间的化学交换作用。K和Rb等LIL元素被明显地从沉积物中去除，而Si则被添加到靠近边界的沉积物中。因此，硅质沉积物的化学特征可以作为玄武岩侵入未固化沉积物的标志。

The results obtained from this research project are summarized as follows.Petrological features and mode of occurrences of greenstones especially exposed in the Sorachi-Yezo belt were reexamined. Geochemical characteristics in respect to minor elemetns, rare earth elements, isotopic compositions of the greenstone, which are previously regarded to have N-MORB signatures, show that they are rather resemble to those of oceanic plateau basalts. In particular, Nb contents are slightly higher than N-MORB whereas HFS elements such as Y and Zr are depleted than N-MORB.Furthermore, intrusive relations of the greenstones against chert were found from several localities, indicating that the igneous activity initiated after the deposition of pelagic sediments. Therefore, these intrusive rocks may correspond to the initial stage of magmatism such as hot-spot type or oceanic plateau type magmatism, Geochemical and age data for the greenstones in the Mikabu belt in Southwest Japan have been carried out also.In situ greenstones with MORB features which were prpbably generated by the subduction of ocean ridge, are extensively described in the shimanto belt and Hidaka belt from Okinawa Island to Hokkaido along thorough Japanese Island. Age determination, mode of occurrence, petrographical and petrological features were studied for them. Systematic younging toward north of radiolarilan fossil ages in the sediments associating with the insitu greenstones bcomes to clear. Besides, interaction between basaltic rocks and intruded sediments was clarified in respect to chemical exchange between them. Some LIL elements such as K and Rb are significantly removed from the sediments whereas Si is added into the sediments near the boundary. Thus, such chmical characteristics of the silicifie sediments can be used as an indicator showing basaltic intrusions into unsolidifed sediments.